Conventionally, JP-A-2004-349398 teaches a thermionic generator for converting thermal energy to electric energy according to phenomena that thermal electron is emitted from a surface of an electrode at high temperature. in order to increase efficiency of generating electricity in the thermionic generator, it is considered that a distance between electrodes is shortened to be a few nano meters so that a tunnel effect occurs.
However, it is difficult to keep the distance between the electrodes to be extremely narrow. When the thermionic generator is manufactured by a mechanical processing method, the above distance may exceed a limit of processing accuracy. Accordingly, US 2003/0184188 and JP-A-2002-540636 teach a method for keeping a distance between electrodes with using a point contact insulator arranged between the electrodes. U.S. Pat. No. 4,373,142 and JP-A-2008-228387 teaches a method for forming a surface of an electrode to be a comb-tooth shape and for forming an insulation layer at a top of the comb-tooth shape.
Further, JP-A-2004-349398 also teaches a method for reducing thermal loss such that a narrow distance between electrodes is uniformly formed by a semiconductor processing technique, and the shortest distance between the electrodes via an insulation spacer is made longer than a distance between the electrodes without the spacer. When the distance between the electrodes is kept by the spacer, the distance can be made extremely narrow since the electrodes are manufactured by the semiconductor processing method, which provides micro fabrication. Further, it is suitable to control the distance stably and to improve reliability. Furthermore, the generator is manufactured at low cost.
However, when the distance between the electrodes is maintained with using the spacers, a surface area of a whole of the spacers increases according to the number of spacers. In this case, a surface resistance of the spacers is reduced, so that current may leak on the surface of the spacers.
Further, it is necessary to reduce an area of each electrode in order to lengthen the shortest distance between the electrodes via the insulation spacer to be longer than the distance between the electrodes without the spacer when the distance between the insulation spacer and the electrode is secured.
Accordingly, the area of the electrode is reduced per unit area of the device, so that the output of the thermionic generator per unit area is lowered.